Surface light source device

ABSTRACT

The present invention has its purpose to obtain a bright luminescent in a surface light source device using a light conducting member. 
     In the surface light source device according to the present invention, a light source is disposed in the vicinity of a light incident edge surface of the light conducting member, a light diffusing plate is arranged on the light emitting side of the light conducting member, and a prism sheet having a multiplicity of convex portions is provided on a side of the light diffusing member, which is opposite to the side where the light conducting member is arranged: and further each of the convex portion of the prism sheet is arranged such that the inclined angle on the light source side is smaller than the inclined angle on the opposite side thereof. Therefore, the luminescent light is directed to the portion to be illuminated of the liquid crystal display apparatus and thus a bright luminescent can be obtained.

TECHNICAL FIELD

The present invention relates to a surface light source device for usein a back light system of a liquid crystal display apparatus.

BACKGROUND ART

FIG. 6 shows a construction of a liquid crystal display apparatus inwhich a conventional surface light source device using a lightconducting member is used as its back light system. In FIG. 6, thenumerical reference 1 represents a plate-like light conducting member,which is made of a transparent material, 2 a linear light sourcedisposed in the vicinity of a light emitting edge surface la of saidlight conducting member 1, 3 a light diffusing plate disposed on a frontsurface 1b of said light conducting member 1 and the numerical reference4 represents a light reflecting member arranged on a rear surface 1c ofsaid light conducting member, constituting of the surface light sourcedevice. Further, the numerical reference 5 represents a liquid crystaldisplay apparatus.

In the surface light source device, a light emitted from the lightsource 2 is made incident upon the light emitting surface 1a of thelight conducting member 1 and then the light is introduced insidethereof. During when the light is transmitted to an opposite edgesurface 1d of the light conducting member, the light is emitted from thefront surface (light emitting surface) 1b of the light conducting memberand then passes through the light diffusing plate 3 to become a diffusedlight. The liquid crystal display apparatus 5 is illuminated by thediffused light coming from the light diffusing plate 3.

The illuminating light generated from the surface light source device isdiffused in all directions by the light diffusing plate 3, but the mostof them is directed to a direction shown by arrows in FIG. 6. Therefore,there exists some light which is not made incident upon the liquidcrystal display apparatus, and thus enough brightness cannot be obtainedon the apparatus. Further, out of the light being made incident upon theliquid crystal display apparatus, only a few light is directed to adirection substantially perpendicular to the liquid crystal displayapparatus, because of the existence of the light diffusion plate 3.Therefore, there is a drawback that the image surface of the liquidcrystal display apparatus is dark for an operator who observes the imagesurface in front of the liquid crystal display apparatus.

In order to dissolve the drawback, it is considered that a prism sheet11, on the surface of which a multiplicity of prism-like portions areformed, is disposed between the light diffusing plate 3 and the liquidcrystal display apparatus 5, and thereby the diffusing light emittedfrom the light diffusing plate 3 is directed to a direction which isperpendicular to the surface of the liquid crystal display apparatus.However, an enough effect cannot be still obtained by using such prismsheet having the prism-like portions, which are formed thereon in asymmetrical manner.

Further, it is also considered that a light diffusing plate 3' having astrong light diffusing power is used as shown in FIG. 7. In such adevice, the light coming through the light diffusing plate 3' isdiffused into directions shown by the arrows in an enough manner, sothat it can be prevented that the large amount of the diffusing light isdirected into a perspective direction as shown in FIG. 6.

However, in this device, since the light diffusing function is strong,the light directed in a perpendicular direction with respect to thelight diffusing plate is relatively decreased, too. Therefore, theproblem cannot be improved that the brightness is not enough for theoperator who observes the image surface in front of the liquid crystaldisplay apparatus. In this case, even if the prism sheet 10 is arrangedin the device, it is impossible to increase the brightness thereof.

The present invention has its object to provide a surface light sourcedevice using a light conducting member, particularly a surface lightsource device having a highly qualified brightness, by which anobservation can be carried out on a bright image surface when thesurface light source device according to the present invention is usedas a back light system of a liquid crystal display apparatus.

DISCLOSURE OF THE INVENTION

A surface light source device according to the present inventioncomprises a light conducting member, a linear light source disposed inthe vicinity of a light emitting edge surface of the light conductingmember, a light diffusing member having a comparatively weak lightdiffusing power disposed on a front surface of the light conductingmember, a light reflecting member disposed on a rear surface of thelight conducting member, and a prism sheet disposed on one side of thelight diffusing member which is opposite to the side on which the lightconducting member is arranged; wherein the prism sheet comprises amultiplicity of prisms on a surface thereof, and each of prism has itscross section, which is in a direction perpendicular to both the lightincident edge surface and the front surface of the light conductingmember, such that an angle of the cross section on the light incidentedge surface side is smaller than an angle on an opposite side thereof.By using the prism sheet having such a construction, the light diffusedby the light diffusing member, which is directed in a perspectivedirection as a whole so as to be separated from the light source, isforced to change its direction into an upper direction. And thus, thebrightness of the surface light source device becomes much higher.

It should be noted that unless the prism-like convex portions formed onthe prism sheet are designed such that the angle on the light incidentedge surface side is smaller than the angle on the opposite side thereofin the above mentioned cross section, any kind of shape of the prismsheet may be possible to use in the device according to the presentinvention and the same effect can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a construction of the firstembodiment of the surface light source device according to the presentinvention;

FIG. 2 is a perspective view depicting a construction of the prism sheetwhich is used in the first embodiment according to the presentinvention;

FIG. 3 is a perspective view illustrating a construction of the prismsheet which is used in the second embodiment according to the presentinvention;

FIG. 4 is a perspective view representing a construction of the prismsheet which is used in the third embodiment according to the presentinvention;

FIG. 5 is a perspective view showing a construction of the prism sheetwhich is used in the fourth embodiment according to the presentinvention;

FIG. 6 is a cross-sectional view depicting a construction of theconventional surface light source device; and

FIG. 7 is a cross sectional view illustrating a construction of anotherconventional surface light source device.

FIG. 8 is a perspective view representing a construction of the prismsheet which is used in yet another embodiment according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the surface light source device according to thepresent invention will be explained in the following. FIG. 1 is across-sectional view showing a liquid crystal display apparatus in whichan embodiment of the surface light source device according to thepresent invention is used; and FIG. 2 is a perspective view illustratinga part of the prism sheet which is used in the first embodiment of thesurface light source device. In these figures, the numerical reference 1represents a light conducting member, 2 a linear light source, 3 a lightdiffusing plate, and the numerical reference 4 represents a lightreflecting member. It should be noted that the surface light sourcedevice has the same construction as those of the devices shown in FIGS.6 and 7. The numerical reference 5 denotes a liquid crystal displayapparatus. In this embodiment, a prism sheet 11 having a constructionwhich is different from the prism sheet 10 shown in FIG. 7 is disposedbetween the light diffusing plate 3 and the liquid crystal displayapparatus 5. The prism sheet 11 has a saw-teeth like convex portions asshown in FIG. 2; that is to say, in a cross section thereof, which iscut in a direction perpendicular to both a light incident edge surface1a and a front surface 1b of the light conducting member 1 (the crosssection illustrated in FIG. 2, which is viewed from the direction shownby the arrow), there are provided a multiplicity of triangles in anorderly manner. It should be noted that the angles A and B of eachtriangle are arranged in such a manner that angle A on the light sourceside (a light incident edge surface side) is smaller than the angle B onthe opposite side.

According to the embodiment, the diffusing light emitted from the lightdiffusing plate 3 as a whole is firstly directed to a direction as shownin FIG. 6, but the direction is changed to an upper direction by thefunction of prisms provided on the prism sheet. Then, the diffusinglight is made incident upon the liquid crystal display apparatus 5. Bythe function of prisms, the most of the diffusing light is directed to adirection which is perpendicular or substantially perpendicular to theprism sheet, so that a bright back light system for the liquid crystaldisplay apparatus, for instance, can be obtained.

FIG. 3 shows a construction of the second embodiment of the prism sheet.The prism sheet 12 comprises prisms each of which has a quadranglepyramid shape, in which an inclined angle of a surface 12a, which isarranged on a light source side, is smaller than that of an oppositesurface 12b. Such a construction, the prism sheet 12 comprisingquadrangle pyramid shaped prisms gets the same function of the prismsheet 11, and thus the same effect can be obtained.

FIG. 4 is a perspective view showing a construction of the thirdembodiment of the prism sheet 13. The prism sheet 13 comprises amultiplicity of conical shaped prisms, in which the top of each prismsis directed to an opposite side of the light source a little. Further,the prisms are arranged in such a manner that an angle corresponding tothe angle A of the prism sheet of the second embodiment, which is shownin FIG. 2, is smaller than an angle corresponding to the angle B of theprism sheet of the second embodiment.

FIG. 5 is a perspective view representing the prism sheet used in thethird embodiment of the surface light source device. The prism sheet 14comprises a multiplicity of prisms each having a quadrangle pyramidshape but without its top portion. Therefore, each of the prism has atrapezoid-like cross section. Further, the inclined angle of theinclined surface 14a of the prism is arranged to be smaller than theinclined angle of the inclined surface 14b, and thus the prism sheet hasthe same function and effect as those of the prism sheet of the secondembodiment.

In the embodiments mentioned in the above, typical shapes of the convexportions (prism portions) formed on the prism sheet are shown. However,the prism sheets may have some shapes other than the above-mentionedembodiments (prism sheets may have prisms having the shapes other thanthose of the embodiments, including the prisms whose corner portions,i.e. apical angle portions or edge portions thereof, are rounded off),and the prism sheets have the same function and effect as those of theabove-mentioned embodiments, if only the prisms formed on the prismsheet are arranged such that the angle on the light source side issmaller than the angle on the opposite side. FIG. 8 illustrates one ofthe above-mentioned shapes, i.e., one in which the apical angle isrounded off.

It is desired that the angle A is less than 45° degrees and the angle Bis between 35° to 60° degrees. That is to say, the angles A and B aredesired to satisfy the following conditions.

A≦45°

35° ≦B≦60°

A<B

In the above mentioned embodiments, the surface light source devicecomprising only one linear light source is shown. However, the presentinvention can be applied to the surface light source device having twoor more linear light sources. For instance, the present invention may beapplied to the surface light source device having two linear lightsources which is obtained by adding one more linear light source on theedge surface 1d side of the light conducting member of the device shownin FIG. 1. In this case, the prism sheet should have its constructionsuch that the prisms formed on the edge surface 1d side of the prismsheet are arranged to be inclined in a direction, which is reversed tothe direction in which the prisms formed on the edge surface 1a side ofthe prism sheet are inclined. (This means in the edge surface 1d side ofthe prism sheet the smaller angle A should be arranged on the edgesurface 1d side and the larger angle B on the edge surface 1a side); andthus the direction of the prisms should be changed at the center of theprism sheet, i.e. between the edge surface 1a and the edge surface 1d.Irrespective of the number of light sources provided in the device, itmay be possible to arrange a plurality of piled-up prism sheets in thedevice. In this case, the prism sheets piled in an upper side from thesecond one may be possible to be designed such that the inclined anglesof the prisms formed on the edge surface 1a side and the edge surface 1dside are the same.

INDUSTRIAL APPLICABILITY

In the surface light source device according to the present invention,the prism sheet having special shaped prisms is arranged on the surfaceof the light diffusing plate so that the most of the light emitted fromthe prism sheet is directed to a direction perpendicular orsubstantially perpendicular to the surface of the prism sheet.Therefore, when the surface light source device according to the presentinvention is used, for instance, as a back light system of the liquidcrystal display apparatus, a bright light source can be obtained.

We claim:
 1. A surface light source device comprising a light conductingmember; a linear light source being disposed in the vicinity of at leastone of light emitting edge surfaces of said light conducting member; alight diffusing member being disposed on a front surface side of saidlight conducting member; a light reflecting member being disposed on arear surface of said light conducting member; and a prism sheet beingdisposed on one side of said light diffusing member, which is oppositeto the side where the light conducting member is arranged, and having amultiplicity of prism-like convex portions on at least one of surfacesof the prism sheet; wherein each convex portion of said prism sheet isarranged such that an inclined angle on a light source side is smallerthan an inclined angle on an opposite side thereof in a cross-section ofthe prism sheet which is cut in a direction perpendicular to both anlight incident edge surface and a front surface of said light conductingmember.
 2. A surface light source device according to claim 1, whereinsaid light source is arranged only one of the edge side surfaces of thelight conducting member.
 3. A surface light source device according toclaim 2, wherein said surface light source device satisfies thefollowing conditions:a≦45° 35° ≦B≦60° A<B wherein the reference symbol Arepresents an angle at a light source side of said convex portion of theprism sheet, and the reference symbol B denotes an angle at an oppositeside of the light source side of said convex portion.
 4. A surface lightsource device according to claim 1, 2 or 3, wherein the convex portionsof said prism sheet form a saw-teeth like shape cross section, when theprism sheet is cut in a direction perpendicular to both the lightemitting edge surface and the front surface of the light conductingmember; and the saw-teeth like convex portions are extended in aperpendicular direction with respect to the cross section.
 5. A surfacelight source device according to claim 1, 2 or 3, wherein each of theconvex portion of said prism sheet has a quadrangle pyramid shape.
 6. Asurface light source device according to claim 1, 2 or 3, wherein eachof the convex portion of said prism sheet has a conical shape.
 7. Asurface light source device according to claim 1, 2, or 3, wherein eachof the convex portions of said prism sheet has a quadrangle pyramidshape without its top portion and thus has its cross section oftrapezoid.
 8. A surface light source device according to claim 1, 2 or3, wherein each of the convex portion of the prism sheet has a conicalor polygonal pyramid shape with a rounded top portion.